In the field of vehicle control, various electronic control systems, such as a keyless entry system and a vehicle condition monitoring system (for instance, tire condition monitoring system), uses a radio communication technique.
A keyless entry system, which is disclosed in JP 2002-129794A for example, includes an in-vehicle unit mounted in a vehicle and a portable unit carried by a user of the vehicle. In this system, when a user carrying the portable unit gets into a detection area within a predetermined range from the vehicle (in-vehicle unit), the portable unit and the in-vehicle unit automatically perform radio communication with each other. When it is authenticated that the portable unit is an authorized one, a predetermined operation (e.g., locking/unlocking doors, starting/stopping an engine, etc.) is automatically performed.
A tire condition monitoring system such as a tire pressure monitoring system (TPMS), which is disclosed in U.S. Pat. No. 5,602,524 (JP-T-7-507513) for example, includes a tire sensor provided in each tire of the vehicle. The tire sensor periodically detects the air pressure, temperature, or the like of the tire and transmits the result of the detection together with identification information for identifying the tire by radio. The in-vehicle unit mounted in the vehicle body receives the result of the detection and informs the vehicle user of the state of the tire or performs other like operation.
When both the keyless entry system and the TPMS are mounted in the same vehicle, each system is required to use a different frequency band to carry out radio communication to prevent radio interference between the systems. It is thus required to provide different radio communication circuits on a system-by-system basis. However, such different communication circuits on a system-by-system basis incur increase in the size and cost of a system configuration. If a radio signal includes noise, radio communication cannot be normally carried out in each system. Especially, if radio communication is not normally carried out in the keyless entry system, it becomes impossible to automatically lock or unlock doors or start or stop the engine.